Pore water chemistry of a Paleogene continental mudrock in Spain and a Jurassic marine mudrock in Switzerland: Sampling methods and geochemical interpretation

Este trabajo resume una serie de metodologías y aproximaciones desarrolladas en CIEMAT durante los últimos 10 años con el objetivo de obtener información del agua intersticial de dos formaciones arcillosas: la Opalinus Clay procedente de Suiza y una formación de edad Oligoceno Mioceno española. Ambas tienen características bastante diferentes tanto desde el punto de vista de la metodología aplicada para su estudio como del origen sedimentario en sí mismo de las dos formaciones. La arcilla Suiza se estudió a través de la participación en el experimento DI-B en el laboratorio subterráneo de Mont Terri, en Suiza, uno de cuyos objetivos fue el muestreo y análisis in situ del agua intersticial de la arcilla. Las muestras de agua se recogieron después de desarrollar equipos y metodologías para conseguir una alteración mínima de los parámetros críticos del agua de la arcilla: pH, Eh, pCO2, Fe(II) y alcalinidad. La modelización geoquímica se utilizó como herramienta para interpretar y entender los principales procesos geoquímicos que controlan la química del agua y el estado actual del sistema. En relación con la arcilla española, el artículo muestra el desarrollo de una metodología integrada para caracterizar el agua intersticial de la formación arcillosa a partir de muestras de testigos correspondientes a un sondeo, denominados S1. El estudio trata de integrar todos los datos disponibles sobre la composición del agua intersticial obtenida mediante la técnica de compresión bajo presión (squeezing) y mediante extractos acuosos, con la caracterización mineralógica y geoquímica de los testigos. La modelización geoquímica se utilizó con dos objetivos principales: para valorar la consistencia de los datos analíticos del agua intersticial y para determinar la importancia relativa de los principales procesos que gobiernan la química del agua intersticial. Específicamente, la modelización se utilizó para simular los procesos que controlan la evolución redox del sistema. Se consideró equilibrio para los minerales reactivos como la calcita, dolomita, yeso y ferrihidrita. Además, se trató la degradación de la materia orgánica y la disolución oxidativa de la pirita desde un punto de vista cinético

In-situ diffusion of HTO, 22Na+, Cs+ and I- in Opalinus Clay at the Mont Terri underground rock laboratory

The diffusion properties of the Opalinus Clay were studied in the underground research laboratory at Mont Terri (Canton Jura, Switzerland) and the results were compared with diffusion data measured in the laboratory on small-scale samples. The diffusion of HTO, Na-22(+), Cs+ and I- were investigated for a period of 10 months. The diffusion equipment used in the field experiment was designed in such a way that a solution of tracers was circulated through a sintered metal screen placed at the end of a borehole drilled in the formation. The concentration decrease caused by the diffusion of tracers into the rock could be followed with time and allowed first estimations of the effective diffusion coefficient. After 10 months, the diffusion zone was over-cored and the tracer profiles measured. From these profiles, effective diffusion coefficients and rock capacity factors Could be extracted by applying a two-dimensional transport model including diffusion and sorption. The simulations were done with the reactive transport code CRUNCH. In addition, results obtained from through-diffusion experiments oil small-sized samples with HTO, Cl-36(-) and Na-22(+) are presented and compared with the in situ data. In all cases. excellent agreement between the two data sets exists. Results for Cs+ indicated five times higher diffusion rates relative to HTO. Corresponding laboratory diffusion measurements are still lacking. However. our Cs+ data are in qualitative agreement wish through-diffusion data for Callovo-Oxfordian argillite rock samples. which also indicate significantly higher effective diffusivities for Cs+ relative to HTO

CO2 Long-term Periodic Injection Experiment at Mont Terri (CO2LPIE).

Peer reviewe

Seasonal variations in pore water and sediment geochemistry of littoral lake sediments (Asylum Lake, MI, USA)

BACKGROUND: Seasonal changes in pore water and sediment redox geochemistry have been observed in many near-surface sediments. Such changes have the potential to strongly influence trace metal distribution and thus create seasonal fluctuations in metal mobility and bioavailability. RESULTS: Seasonal trends in pore water and sediment geochemistry are assessed in the upper 50 cm of littoral kettle lake sediments. Pore waters are always redox stratified, with the least compressed redox stratification observed during fall and the most compressed redox stratification observed during summer. A 2-step sequential sediment extraction yields much more Fe in the first step, targeted at amorphous Fe(III) (hydr)oxides (AEF), then in the second step, which targets Fe(II) monosulfides. Fe extracted in the second step is relatively invariant with depth or season. In contrast, AEF decreases with sediment depth, and is seasonally variable, in agreement with changes in redox stratification inferred from pore water profiles. A 5-step Tessier extraction scheme was used to assess metal association with operationally-defined exchangeable, carbonate, iron and manganese oxide (FMO), organic/sulfide and microwave-digestible residual fractions in cores collected during winter and spring. Distribution of metals in these two seasons is similar. Co, As, Cd, and U concentrations approach detection limits. Fe, Cu and Pb are mostly associated with the organics/sulfides fraction. Cr and Zn are mostly associated with FMO. Mn is primarily associated with carbonates, and Co is nearly equally distributed between the FMO and organics/sulfide fractions. CONCLUSION: This study clearly demonstrates that near-surface lake sediment pore water redox stratification and associated solid phase geochemistry vary significantly with season. This has important ramifications for seasonal changes in the bioavailability and mobility of trace elements. Without rate measurements, it is not possible to quantify the contribution of various processes to natural organic matter degradation. However, the pore water and solid phase data suggest that iron reduction and sulfate reduction are the dominant pathways in the upper 50 cm of these sediments